There are devices presently available to detect acoustic signals from the chest each having its own advantages and disadvantages as described in the review by Semmlow and Rahalkar. The motivation for most of these devices is the detection of sound signatures associated with coronary artery disease as originally described by Semmlow el al. in 1983. Coronary artery disease results from occlusions or blockages of the coronary arteries which supply blood to the heart. Such blockages will produce turbulent blood flow including an auditory correlate. Theoretical studies by Wang et al indicate that said auditory correlates will be at relatively high frequencies: above 200 Hz and as high as 1 kHz. Such sounds are generally too faint and at too high a frequency to be heard through a traditional stethoscope, although murmurs associated with coronary artery disease have occasionally been reported. Acoustic detection of the sounds produced by blood flowing through partially occluded coronary arteries would thereby enable the noninvasive detection of this major disease. An accelerometer-based sensor described by Padmanahban et al. and in U.S. Pat. Nos. 5,036,857 and 5,109,863 has produced signals that were moderately successful in detecting coronary artery disease as shown by Akay et al. Semmlow (U.S. Pat. Nos. 9,226,726 and 9,320,489) describes a similar sensor and system for detecting coronary artery disease noninvasively based on sounds detected from the chest.
The quality of the signals acquired from the chest has a direct impact on the reliability of the diagnosis information derived from that signal. The quality of the signal produced by a cardiac microphone will also depend on microphone position and attachment to the chest along with patient factors such as body weight. Hence the quality of signals produced by any detector will vary from patient-to-patient and even measurement-to-measurement. There is therefore a clear need for a detector which provides quantitative information on the quality of the acquired signal over the range of desired frequencies. The present invention accomplishes these objectives.
Semmlow (U.S. Pat. No. 9,226,726) describes a method based on correlation for determining the signal-to-noise ratio (SNR) given two copies of a signal and associated noise. The present invention is an improved method for evaluating SNR from two signal copies based only on algebraic analysis. This method may be faster to implement on a computer and can provide a more accurate estimate of SNR. It is more robust to imbalances in the two signals and to differences in the noise components of the two signals. The present invention also produces an estimate of both the signal and noise components in addition to the SNR.